Weighing Single Atoms?

Recent research in nanoelectromechanical systems (NEMS) has focused on tiny RF oscillators built from silicon rods.

Weighing single atoms?

But replacing the silicon rod with a carbon nanotube let Cornell University researchers shrink the oscillators even further and make them more durable and sensitive to mass changes. A tiny electromechanical-oscillator "scale" built with carbon nanotubes may be capable of weighing a single atom, say researchers.

Carbon nanotubes are cylinders of carbon atoms arranged in a hexagonal pattern similar to that in the geodesic domes created by architect, inventor, and mathematician Buckminster Fuller. Materials with this structure are called fullerenes in his honor, and fullerene spheres are dubbed " buckyballs." A nanotube can be thought of as an elongated buckyball.

The Cornell device consists of a carbon nanotube about 1 to 4 nm in diameter and 1.5 m long, suspended between two electrodes above a silicon plate. A nanometer is roughly the length of three silicon atoms side by side. The tube hangs like a chain between two posts in a curve called a catenary. A voltage potential between the tube and underlying plate creates a mutual electrostatic force: An ac voltage applied to the plate causes the tube to vibrate, while a dc voltage on the tube proportionally increases tension and vibration frequency. Current flow is an indicator of tube motion. Researchers have used the technique to make oscillators that tune over a range from 3 to 200 MHz. Such a tunable oscillator could be used as a detector in cell phones, which must constantly change operating frequency to avoid conflicts with other phones.

Mass sensing is another potential application for nanotube oscillators. Classical physics says the frequency of a vibrating "string" also depends on its mass. Silicon-rod oscillators use this property to weigh bacteria and viruses, for example. Nanotubes, with their smaller mass, should have even greater mass sensitivity, high enough to perhaps weigh individual atoms, say researchers. But it could be a while before practical devices hit the market: There is currently no way to mass-produce carbon nanotubes.

Funding for the work comes from the National Science Foundation and the Microelectronics Advanced Research Program Focus Center on Materials, Structures and Devices supported by the Semiconductor Research Corp. The devices were fabricated at the NSF-funded Cornell Nanoscale Facility.